Find $a$ such that $ax^2+15x+4$ is the square of a binomial.
Answer: The square of the binomial $rx+s$ is  \[(rx+s)^2=r^2x^2+2rsx+s^2.\]If this is equal to $ax^2+15x+4$, then $s$ must be either 2 or -2.  Since $(rx+s)^2=(-rx-s)^2$, we may choose either $s=2$ or $s=-2$, and the solution will be the same.  We choose $s=2$.

The square of $rx+2$ is  \[(rx+2)^2=r^2x^2+4rx+4.\]If this is equal to $ax^2+15x+4$ then we must have $15=4r$ or $r=\frac{15}4$.  This gives our square: \[\left(\frac{15}4x+2\right)^2=\frac{225}{16}x^2+15x+4.\]Therefore $a=\boxed{\frac{225}{16}}$.